scabbard-fish

from The Century Dictionary.

• noun A fish of the family Lepidopodidæ, Lepidopus caudatus, of the Mediterranean and Atlantic shores of Europe, as well as of New Zealand, of a bright silvery color, with a long dorsal and rudimentary anal fin: so called from suggesting by its form the sheath of a sword. Also called scale-fish and frost-fish.
• noun Any fish of the family Gempylidæ.